In a multi-cell time division duplex (TDD) communication system, channel state information (CSI) is obtained by inferring based on a pilot signal from a user device. To obtain the CSI, each user device needs to transmit a pilot signal to a base station that the user device belongs to. The pilot signal refers to sequences known at a transmitting terminal (e.g., user device) and a receiving terminal (e.g., base station), and different sequences are orthogonal to each other. After receiving different pilot signals, the receiving terminal may distinguish the pilot signals from different transmitting terminals based on the orthogonality of the pilot signals with respect to each other, so as to respectively estimate channels for different transmitting terminals. From the perspective of linear algebra, the total number of orthogonal pilot signals in a set is equal to the length of the pilot signal. Thus, if the length of the pilot signal is enough, the pilot signals orthogonal to each other may be assigned to all the transmitting terminals.
In the block fading channel model, a channel coefficient remains as a constant in a limited time period referred to as coherent time block, and is changed in the next coherent time block. The coherent time block is generally divided into: (1) training time; and (2) data transmission time. The user device needs to transmit the assigned pilot signal in the training time of each coherent time block to allow the base station to update the current CSI accordingly. When the moving speed of the user device increases, the channel environment also changes drastically. Therefore, the length of the coherent time block is correspondingly shortened.
Clearly, the length of the pilot signal must be limited, or the throughput will be lowered due to an overly short data transmission time. When the length of the pilot signal is limited, the pilot signals are not enough to be assigned to the enormous user devices in the communication system. Thus, the pilot signals must be reused in different cells. Under the circumstance that the user devices in different cells use the same pilot signals, the pilot signals transmitted by these user devices will cause significant inter-cell interference (ICI), which is also referred to as pilot contamination. Under such circumstance, the channel estimated by each base station may be contaminated due to ICI, making the result of channel estimation inaccurate. Moreover, the subsequent data transmission performance is affected, too.
For example, when the base station is unable to correctly estimate the channel to each user device, the base station is unable to correctly calculate the downlink beam former. Under such circumstance, the beam former of the base station may be unable to directly align the signal with the user device suffering from pilot contamination. Thus, the data rate of the user device is limited.
Thus, for researchers in this field, how to effectively alleviate or prevent the pilot contamination between the user devices using the same pilot signals has become an issue to work on.
Even though there are several methods against pilot contamination in the conventional art, these methods require geographical locations of the user devices in advance, so as to assign the same pilot signal to the user devices relatively distant from each other to thereby reduce ICI. In addition, as shown in FIG. 1, the conventional art also includes suitably scheduling the time points when the user devices transmit/not transmit the pilot signals and performing a specific signal processing method to eliminate ICI.
Referring to FIG. 1, FIG. 1 is a view illustrating a conventional process of eliminating ICI. In FIG. 1, it is assumed that the network cluster being considered includes seven cells, and the user devices in the cells use the same set of pilot signals (i.e., the reuse rate of pilot signals is 1/7), and the training time is divided into 8 time slots. In FIG. 1, if a field corresponding to the p-th (p is an integer not greater than 7) cell and the q-th (q is a positive integer not greater than 8) time slot is indicated as “+1”, the user device in the p-th cell transmits the assigned pilot signal at a transmission power of one unit at the q-th time slot. Alternatively, if the field corresponding to the p-th cell and the q-th time slot is indicated as “0”, it is indicated that the user device of the p-th cell does not transmit the assigned pilot signal at the q-th time slot.
Then, after the training time, the base station may perform a process of eliminating ICI on the user devices in the respective cells based on FIG. 1. Specifically, the base station of the first cell may subtract the pilot signal received at the second time slot from the pilot signal received at the first time slot, so as to eliminate the ICI resulting from the second to seventh cells. As another example, the base station of the second cell may subtract the pilot signal received at the third time slot from the pilot signal received at the first time slot, so as to eliminate the ICI resulting from the first and third to seventh cells. After ICI is eliminated, the base station may estimate the wireless channel between the user device and the base station by using an estimator such as a least square (LS) estimator, etc. However, even though the mechanism shown in FIG. 1 is able to eliminate ICI, the transmission power in such mechanism is only 1 unit. Thus, the corresponding signal to noise ratio (SNR) is not high in practice. Under such circumstance, the accuracy of wireless channel estimation is not preferable.